Outlet arrangement for down-hole separator

ABSTRACT

A method and an arrangement for separating oil and water by means of a two-phase liquid/liquid separator. The oil phase and the water phase respectively are separated over a portion of the length of the separator from an upstream end to a downstream end. The oil phase and the water phase respectively are extracted gradually over a length of the separator, and each of the phases is conducted out of the separator separately. A method and an arrangement for orienting the separator in a well are also described.

RELATED APPLICATION

This application is a National Phase entry in the United States of theInternational Application PCT/NO01/00156 filed Apr. 09, 2001 and claimsthe benefit of the Norwegian application number 2000 1954 filed Apr. 13,2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention regards an outlet arrangement for use in separation ofwater and oil by means of a two-phase liquid/liquid separator in whichthe oil and water phases are respectively separated over at least partof the length of the separator from an upstream end to a downstream endof the separator. In particular, the invention regards an outletarrangement for a horizontal gravity separator with a highlength/diameter ratio. The invention especially regards a downholeseparator for placing in an underground well. The invention also regardsa method for inhibiting re-mixing of the oil and water phases in aseparator where the separator includes a separator chamber with anupstream end and a downstream end and an oil outlet and a water outlet,the water outlet being situated in an upper part of the separatorchamber and the oil outlet being situated in a lower part of theseparator chamber.

The invention also regards a method for orienting an outlet arrangementin a horizontal downhole separator, as well as means of implementingthis orientation.

2. Description of the Related Art

Separation of well fluids is usually carried out by use of a 3-phasegravity separator on a platform. Typically, the vessels have adiameter/length ratio in the range of 1:3 to 1:5. The separator isusually partially filled with gas, so as to provide two interfaces; agas-liquid interface and an oil-water interface. Water and oil areextracted by means of a suitably elevated, vertically oriented pipe witha vortex breaker. The pipes are typically provided in the lower part ofthe vessel, at the opposite end from the fluid inlet. A weir plate isoften used to isolate the water phase from the oil outlet. In this case,the oil-water interface is kept below the level of the weir plate, sothat only the oil phase will flow over this.

In a horizontal tube or a vessel, the oil and water are separated bygravity due to the difference in density between the two phases, so asto form a lighter phase and a heavier phase. A stratified flow patternmust be maintained in order to be able to separate the liquid phases.The cross-sectional flow area is sized with respect to fluid velocityand water cut. If the residence time is sufficiently high to transferdroplets from the continuous phases, clean oil and water can beproduced. The oil droplets will rise from the water phase, and if givensufficient residence time, they will reach the oil-water interface.Water droplets in the oil phase will settle and eventually reach theinterface. Accumulated droplets of oil and water will form a dense layerin the interfacial region, in which coalescence takes place. If theresidence time is sufficient, the dense layer will eventually be brokenup.

Gas may be separated out from the well fluid by use of one or morecyclone separators. The water may also be cleaned by use ofhydrocyclones. In a multi-stage separation system consisting of suchseparators, the main aim is to provide a clean oil phase. This is bestachieved by employing a long, slim separator geometry in order tomaximise the oil-water interface where the coalescence of the dropletstakes place. Furthermore, the distance to the oil-water interface,across which distance the droplets have to travel, is reduced.

A downhole oil/water gravity separator in which the separation takesplace in a section of the production tubing in a horizontal well, suchas disclosed in WO 98/41304, is a separator with a very high length todiameter ratio. A separator of this type will typically have a length ofapproximately 100 m, while the diameter is the same as for theproduction tubing, for instance 9″, 10¾″ or 13″, i.e. a length-diameterratio of 1:300 to 1:400.

A large oil-water interface and sufficient residence time alone is notenough to achieve clean oil and water phases at the outlet of theseparator. It is crucial that the separation of oil and water take placewithout either of the phases becoming contaminated by the other uponextraction from the separator.

Since the diameter of the outlet is small, this will easily influencethe oil-water interface and thereby result in water entering the oilphase and oil entering the water phase.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to ensure that eachof the phases is distributed to the appropriate outlet withoutinterfering with the oil-water interface and re-mixing the phases. Asthe phases exist in a separated state as much as several meters beforethe end of the separator, each of them should be extracted graduallyover a certain distance before the end wall of the separator. Throughsuch gradual extraction of separated fluids, the residence time for theremaining liquid in the separator will be extended, allowing the smalleroil and water droplets to migrate to the oil and water phasesrespectively. This is achieved by a two phase liquid/liquid separator inwhich the oil and water phases respectively are separated over at leastpart of the length of the separator from an upstream end to a downstreamend of the separator wherein the oil and water phases are respectivelyare extracted gradually over a length of the separator, the lengthextending substantially all the way from the downstream separator andwherein the oil and water phases respectively are extractedsubstantially symmetrically respective to longitudinal axis of theseparator and wherein each of the phases is extracted from the separatorseparately. This is further achieved by an arrangement at a separatorfor separating oil from water, the arrangement including a separatorchamber with an upstream and a downstream end and an oil outlet and awater outlet, the water outlet being situated in an upper part of theseparator chamber and the oil outlet being situated in a lower outlet ofthe separator chamber wherein the oil and water outlet are respectivelydistributed over a length of the separator at the downstream end of thisand wherein the oil and water outlets, with respect to each other, arearranged substantially symmetrically relative to the longitudinal axisof the separator.

In the case of an outlet arrangement of the type with which the presentinvention is concerned, it is important for the separator to have thecorrect orientation, so that the oil outlets face upwards and the wateroutlets face downwards. According to the present invention, a method andan arrangement have therefore been provided in order to achieve this.The method is characterised by orienting an outlet arrangement in ahorizontal downhole separator such that a guide device is inserted intoa hole and fixed relative to the hole, inserting a measuring tool intothe hole in order to register the orientation of first guide means onthe guide device, setting second guide means on the separator, thesecond guide means corresponding to the first guide means, in accordancewith the orientation of the first guide means, inserting the separatorwith the outlet arrangement into the hole, and rotating the separator toa predetermined lintation upon contact between the first and secondguide means. When oil and water are extracted from the separator over acertain distance, the effect on the interface is lessened. It becomeseasier to avoid extracting liquid from the area near the interface.Small, dispersed droplets of the opposite phase will often be presentnear the interface, e.g. water droplets in the oil phase and oildroplets in the water phase. These droplets will be given more time tocoalesce and migrate to the phase to which they belong.

The outlet arrangement according to the present invention also aims tohave a design which is such that oil and water are gradually extractedin a radial direction from the upper and the lower part of the separatorhousing, near the separator wall.

By the arrangement according to the invention, the oil and water outletsrespectively are distributed over a certain axial distance at thedownstream end of the separator.

By such an arrangement, clean oil and water may be collected as early aspossible upon becoming accessible, and then be transported out of theseparator via the outlet pipe. Any disturbance of the liquid phases willbe minimal when oil and water are removed over an extended distance. Byconstructing outlet slots or holes with a decreasing cross sectionalarea in the downstream direction, the velocity of the liquid in theoutlet pipe will increase in the downstream direction. The accelerationis preferably linear. When the clean oil and water are brought out at ahigher velocity than the mean bulk velocity in the separator, the totalresidence time is increased, making it easier for smaller droplets tocoalesce and migrate.

Heavier solids will have time to settle in the separator, while lighterparticles will be carried along by the liquid phases. By the arrangementaccording to the present invention, it is also possible to avoid solidsaccumulating near the outlet and blocking this. By extracting water nearthe bottom of the separator, the liquid flow will also sweep away solidsnear the bottom and carry these out with the water.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained by means of an exampleof an embodiment of an arrangement and a separator in accordance withthe present invention, with reference to the accompanying drawings, inwhich:

FIG. 1 is a perspective drawing showing an outlet arrangement inaccordance with the present invention;

FIG. 2 is a longitudinal section of a part of a separator according tothe present invention, including the outlet arrangement shown in FIG. 1;

FIGS. 3 a -c are sections along the lines A—A, B—B and C—C in FIG. 2respectively;

FIG. 4 shows a separator equipped with guide means for orienting theseparator;

FIG. 5 shows a guide sleeve to be used when orienting the separator; and

FIG. 6 shows a detail of a gear rim that forms part of the guide means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2 and 3 a, b, and c show a longitudinal and transverse sectionsrespectively through part of a liquid-liquid separator according to thepresent invention and including the outlet arrangement shown in FIG. 1.The separator has a high length to diameter ratio.

The separator comprises a separator wall 1 tubularly enclosing aseparator chamber 20. An outlet arrangement 2 is provided inside theseparator chamber 20. The outlet arrangement 2 is positioned along thelongitudinal axis of the separator chamber 20, and extends over a lengthof e.g. 2-4 meters at the downstream end of the separator.

The outlet arrangement 2 has an outer end 21 facing the upstreamdirection and an inner end 22 that is connected to transport conduitsoutside the separator (not shown). The outlet arrangement furthercomprises an outer wall 23 that forms a pipe having approximately thesame cross section along the length of the outlet arrangement 2. Anannulus 10 is defined between the separator wall 1 and the outer wall23. First inlet orifices 4 a are formed in the outer wall 23, on oneside of the outer wall 23, and second inlet orifices 4 b on the oppositeside of the outer wall. As shown for orifices 4 a in FIG. 1, theorifices 4 a and 4 b have a decreasing cross section from the outer end21 of the outlet arrangement 2 to the inner end 22. In the embodimentshown, this has been accomplished by reducing the width of theinwardlying orifice 4 a, 4 b in relation to the outwardlying orifice 4a, 4 b.

The outlet arrangement 2 includes an inner wall 5 that is concentricwith the outer wall 23, which inner wall 5 also extends tubularly with acircular cross section along the longitudinal axis of the separator. Anannulus 32 is defined between the outer wall 23 and the inner wall 5.

As shown in FIG. 3 a, four longitudinal walls 24, 25, 26 and 27 areprovided between the outer wall 23 and the inner wall 5, which wallsextend along most of the length of the outlet arrangement 2. A channel 3a is defined between walls 24 and 25, the outer wall 23 and the innerwall 5. A channel 7 is defined between walls 25 and 26, the outer wall23 and the inner wall 5. A channel 3 b is defined between walls 26 and27, the outer wall 23 and the inner wall 5. A channel 6 is definedbetween walls 27 and 24, the outer wall 23 and the inner wall 5.

Near the outer end 21, the walls 24, 25, 26 and 27 become a cone 28.Near the inner end 22, the walls 24, 25, 26 and 27 become twolongitudinal walls 29 and 30 (see FIG. 3 b) extending between the outerwall 23 and the inner wall 5. Above walls 29 and 30 and between theouter wall 23 and the inner wall 5, there is defined a channel 12. Belowwalls 29 and 30 and between the outer wall 23 and the inner wall 5,there is defined a channel 11.

The walls 24, 25, 26 and 27 and the walls 29 and 30 all have a point ofintersection at the longitudinal axis of the separator.

Even closer to the inner end 22, the walls 29 and 30 deflect upwards andconverge to form a sloping wall 13. The sloping wall 13 closes off thechannel 12 above the walls 29 and 30.

In the part of the outer wall 23 that is located outside of the channels6 and 17, a plurality of orifices 33 are formed so as to let the annulus10 communicate freely with the channels 6 and 7. The separator chamberavailable for performing separation outside of the outlet arrangement istherefore the combination of the annulus 10 and the channels 6 and 7,between which there is fluid communication. In principle, the portionsof the outer wall 23 located outside of the channels 6 and 7 have noother function than to ensure that the outlet arrangement achievesgreater rigidity.

Immediately outside of the sloping wall 13, a plurality of packings 8have been provided in the annulus 10, which packings 8 form a liquidtight end of the separator chamber 20.

An opening 9 is formed in the outer wall 23 between the packings 8 andthe sloping wall 13. This opening 9 connects the channel 3 a with anannulus 34 located inside the packings and between the separator wall 1and the outer wall 23. Beyond the sloping wall 13, an annulus 32 isdefined between the outer wall 23 and the inner wall 5.

A channel 35 is defined within the tubular inner wall 5.

The functioning of the separator and the outlet arrangement according tothe invention will now be explained.

Water and oil flow in towards the outlet arrangement 2 from the left inFIG. 2. The water and the oil are then already more or less completelyseparated. The oil indicated by arrows 36 will settle at the top of theseparator and flow in through the orifices 4 a in the upper side of theouter wall 23. The water indicated by arrows 37 will settle at thebottom of the separator and flow in through the orifices 4 b in theunderside of the outer wall 23.

A local drop in the velocity of the liquid phases will occur at theouter end 21 due to the cone 28 that causes a narrowing of the crosssection.

The oil will flow on through channel 3 a at a velocity that increases asthe flow nears the inner end 22 of the outlet arrangement 2. Most of theoil will flow into channel 3 a through the orifices 4 a closest to theouter end 21 of the outlet arrangement and to a lesser extent near theinner end 22. The oil continues further through channel 3 a and channel12, and out through the opening 9 and into the annulus 34, from which itis transported to the surface.

Likewise, the water will flow on through channel 3 b at a velocity thatincreases as the flow nears the inner end 22 of the outlet arrangement2. Most of the water will flow into channel 3 b through the orifices 4 bclosest to the outer end 21 of the outlet arrangement and to a lesserextent near the inner end 22. The water flows from channel 3 b and intochannel 11. Immediately beyond the sloping wall 13, the water is allowedto fill the entire annulus 32. From here, the water may be transportedto the surface, or it may be sent back via channel 35 by means of a pump(not shown), to be injected into the formation. If injection is not anissue, this pipe may be replaced by partitions.

In the case of an outlet arrangement of the above mentioned type, it isimportant that this is oriented correctly in the separator, so as toleave the orifices 4 a facing upwards and the orifices 4 b facingdownwards. Even though the outlet arrangement can tolerate a certainamount of deviation from this, the performance of the outlet arrangementwill deteriorate with any major deviation from this orientation. Belowis described an example of an embodiment of the present invention, whichembodiment ensures the correct orientation of the outlet arrangement.

FIGS. 4 and 5 show devices for orienting the outlet arrangementaccording to FIGS. 1-3. FIG. 4 shows a separator according to thepresent invention. The separator wall 1 is shown furthest out. Theoutlet arrangement 2 is placed inside this. The end 22 of the outletarrangement faces left in the Figure. Here, several openings 9 areshown. The channel 35 in the pipe 5 is also shown.

On the outside of the separator is provided lock nuts 40 and 41 for apacker (not shown). A gear rim 42 equipped with a key 43 is alsoprovided. The key 43 is a wedge that may be moved along the periphery ofthe gear rim 42 and locked into this. This may for instance beimplemented by placing the key 43 in one of several slots in the toothedring 42. A locking ring 56 is also provided in connection with the gearrim 42, in order to lock the gear rim 42 to the separator.

FIG. 6 shows the gear rim 42 with wedge 43 and key slot 57 in detail.The key slots are spaced at angular interval of e.g. 5°.

FIG. 5 shows a guide sleeve 50. A wedge belt 51 is provided about theperiphery of the guide sleeve 50. This wedge belt 51 may have aconventional construction, and is designed to fix the guide sleeve 50 inthe casing (not shown). At one end 52, the guide sleeve 50 has beenbevelled so as to form a guide edge 53 and 54. The guide edges 53 and 54extend in towards a guideway 55 that extends along a length of the guidesleeve.

When the separator is to be placed in the well, the guide sleeve 50 isfirst inserted into the hole with the end 52 facing upwards, until itreaches a horizontal part of the well, in which part the separator isrequired to be placed. Upon reaching this site, the wedge belt 51 isactivated so that the guide sleeve is immobilised against the casing.Then a monitoring tool of a type that is known per se is inserted intothe well in order to register the direction in which the guideway 55 hasbeen oriented. This measurement determines the positioning of the key 43along the periphery of the gear rim 42.

The key is then placed at the same angle relative to the inlet orifices4 a as that formed by the guideway 55 in relation to the upwarddirection. The separator is then inserted into the well. They key 43contacts one of the guide edges 53, 54. As the separator is insertedinto the guide sleeve, the guide edge 53 or 54 causes the separator torotate until the key 43 is flush with the guideway 55. When the key 43is inserted into the guideway 55, the separator has the correctorientation. Finally, the separator is fixed by means (not shown) thatare know per se, and the remaining required pipes are connected to it.

Instead of the key being located on the separator and the guide surfaceswith the guideway being provided on a guide sleeve, it is also possibleto provide the separator with guide surfaces and a guideway, and fixinga device comprising a key in the casing.

1. A method of separating oil and water with a two-phase liquid/liquidseparator, in which the oil and water phases respectively are separatedover at least part of the length of the separator from an upstream endto a downstream end of the separator, wherein the oil and water phasesrespectively are extracted gradually over a length of the separator, thelength extending substantially all the way to the downstream end of theseparator, and wherein the oil and water phases, respectively, areextracted substantially symmetrically relative to the longitudinal axisof the separator and wherein each of the phases is extracted from theseparator separately.
 2. The method according to claim 1, wherein eachphase is substantially evenly extracted over said length of theseparator.
 3. The method according to claim 1, wherein accumulatedsolids are carried out of the separator together with the separatedwater phase.
 4. An arrangement at a separator for separating oil andwater, comprising a separator chamber with an upstream end anddownstream end and an oil outlet and a water outlet, the water outletbeing situated in a lower part of the separator chamber and the oiloutlet being situated in an upper part of the separator chamber,characterised in that the oil and water outlets respectively aredistributed over a length of the separator at the downstream end of thisand that the oil and water outlets with respect to each other arearranged substantially symmetrically relative to a longitudinal axis ofthe separator.
 5. The arrangement according to claim 4, characterised inthat the oil and water outlets respectively comprise orifices with agradually decreasing cross sectional area from the upstream end to thedownstream end of the separator.
 6. The arrangement according to claim 4wherein the oil outlet is located near an upper boundary of theseparator chamber and the water outlet is located near a lower boundaryof the separator chamber.
 7. The arrangement of claim 4, wherein the oiland water outlets respectively are included in an outlet arrangementhaving an outer wall and at least one longitudinal wall defining an oiloutlet channel and a water outlet channel.
 8. The arrangement accordingto claim 7, characterised in that the oil outlet channel is defined by afirst portion of any annulus between the outer wall and an inner wall,and that the water outlet channel is defined by a second portion of anannulus between the outer wall and the inner wall.
 9. The arrangement ofclaim 7, wherein the oil and water outlet channels respectively aredesigned so as to position the oil outlet generally at the top and thewater outlet generally at the bottom with respect to vertical, andgenerally at the downstream end of the separator chamber.
 10. Thearrangement according to claim 9, wherein, for orienting the outlet inthe vertical plane, the outlet arrangement is rotated to a desiredorientation during insertion into a guide sleeve with a guideway thatgives the desired orientation in a rotational plane.
 11. The arrangementaccording to claim 8, characterised in that at least one packing isprovided between the separator wall and the outer wall in order to forma liquid tight end of the separator chamber.
 12. The arrangementaccording to claim 11, characterised in that the oil outlet channelcommunicates with an oil outlet annulus downstream of the at least onepacking, which annulus is defined between the separator wall and theouter wall, and that the water outlet channel communicates with a wateroutlet annulus downstream of the at least one packing, which annulus isdefined between the outer wall and the inner wall.
 13. The arrangementaccording to claim 8, characterised in that a channel defined inside theinner wall is designed to transport water for water injection.
 14. Thearrangement claim 4, wherein the oil and water outlet, respectively, aredistributed over a length of 2-4 meters.